Recently, plastics such as polyethylene terephthalate (PET) or like polyester resins, acrylic resins, ABS resins, polycarbonate resins, and fiber reinforced plastics (FRP) have been used in a wide range of products including exterior building components and exterior parts for automobiles, because of their lightweightness, impact resistance, processability, and recyclability. However, it is pointed out that these synthetic resins have a drawback in that yellowing and cracks can be seen when used outside over a long period of time, and a problem in that their surfaces are prone to be scratched as compared to glass and metallic materials.
To overcome these issues, there is a method in which the weather resistance is given to plastics by protecting their surfaces with coatings or the like. As to the coatings for use in this application, polysiloxane-based coatings, fluoroolefin-based coatings, and the like, have been reported so far. Polysiloxane-based coatings which do not emit toxic halogen gas when burned have been preferably used in a variety of fields such as exterior building works.
As a curable resin composition for use in such polysiloxane-based coatings, the inventors of the present invention have previously invented and disclosed a curable resin composition including: a resin prepared by condensing a polymer having both of a hydrolyzable silyl group and a specific functional group other than hydrolyzable silyl groups, with a specific polysiloxane having a silanol group and/or a hydrolyzable silyl group; and a curing agent (for example, refer to Patent Document 1). Cured coating films made of this curable resin composition are excellent in weather resistance and scratch resistance.
However, because this curable resin composition is thermally curable, it needs to be heated to as high a temperature as 140° C. so that excellent scratch resistance can be given to the cured coating film (refer to Example 1 of Patent Document 1). Accordingly, there is a problem in that when applied to a plastic material which is usually heat sensitive, the base material becomes deformed or discolored by heat.
To overcome these issues, the inventors of the present invention have invented and disclosed a UV-curable polysiloxane coating as a system that can be cured without high temperature heating (for example, refer to Patent Document 2). Specifically, it is a UV-curable coating including: a composite resin which includes a polysiloxane segment having a silanol group and/or a hydrolyzable silyl group as well as having a polymerizable double bond, and which also includes a polymer segment other than the polysiloxane segment; and a photopolymerization initiator. Because there are two hardening mechanisms, namely, UV curing and crosslink densification of the coating film through the condensation reaction of silanol groups and/or hydrolyzable silyl groups, it becomes possible to produce cured coating films which excel in scratch resistance, acid resistance, alkali resistance, and solvent resistance, and it also becomes possible to suitably use this coating for application to base materials, such as exterior coatings for buildings and plastics, which have been so far difficult to use with thermally curable resin compositions as they are susceptible to heat deformation.
However, this UV-curable coating is not expected to be used under quite severe conditions that are equivalent to exposure to the outside over a long period as long as ten or more years. For example, there has been revealed a problem in that a cured coating film produced by the method described in Example 1 became cracked after an accelerated weather resistance test that corresponded to ten years exposure to the outside. Furthermore, another problem has also been revealed in that, for example, when a cured coating film produced by the method described in Example 3 was directly formed on a polyethylene terephthalate (PET) base material or a polycarbonate base material, the adhesion between the base material and the cured coating film was considerably deteriorated after a severe humidity resistance test.
On the other hand, as a method for giving weather resistance and scratch resistance to the outermost layer made of a synthetic resin that is relatively resistant to heat deformation such as a polycarbonate resin, for example, there is disclosed a method in which a polycarbonate base material is coated with an acrylic resin as a first layer and a thermally curable coating film layer having a specific organosiloxane resin composition is coated thereon as a second layer (for example, refer to Patent Document 3).
However, this method is difficult to apply to a synthetic resin that is quite susceptible to heat deformation such as polyethylene terephthalate (PET), and also a problem is pointed out in that the outermost layer is fractured at the time of a bending operation.    Patent Document 1: International Publication No. WO/1996/035755 Pamphlet    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2006-328354    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2004-026871